Composition of matter

ABSTRACT

A composition of matter comprising: 5 wt % to 8 wt % rhenium; 4 wt % to 8 wt % tantalum; 2 wt % to 5 wt % tungsten; 2 wt % to 5.5 wt % molybdenum; 2 wt % to 5 wt % chromium; 2 wt % to 6 wt % ruthenium; 2 wt % to 8 wt % cobalt; 5 wt % to 7 wt % aluminium; 0 wt % to 2 wt % titanium; 0 wt % to 0.5 wt % hafnium; and the balance nickel and incidental impurities.

FIELD OF THE INVENTION

This invention relates to compositions of matter. More particularly, butnot exclusively, this invention relates to nickel based alloys, such asnickel based superalloys. Embodiments of the invention relate tonickel-based single crystal superalloys.

DESCRIPTION OF THE PRIOR ART

To improve the performance and efficiency of gas turbine engines, singlecrystal Ni-base superalloy turbine blades have been increasingly alloyedwith dense refractory elements to enhance high temperature creepproperties. As single crystal compositions have become more heavilyalloyed, the ease of manufacturing and processing has decreased,primarily due to formation of solidification related grain defects.Moreover, the recent trend of adding ruthenium to single crystal Ni-basesuperalloys has led to substantial increases in raw material costs, thusmaking manufacturing yield improvements through the reduction of castingdefects of paramount importance. Hence, assessment of the solidificationcharacteristics of these complex multi-component alloys andunderstanding the various elemental interactions is critical during thedevelopment of advanced single crystal Ni-base superalloys with improvedhigh temperature properties and long term stability.

In multi-component alloys such as nickel-base superalloys,solidification involves solute redistribution of the alloying elementsduring dendrite growth. This causes microsegregation i.e. localvariations in elemental concentrations from the dendrite cores to thedendrite peripherals and interdendritic region of the as-cast alloy. Insingle crystal Ni-base superalloys, solidification begins with theformation of primary γ-Ni dendrites and typically terminates at a γ+γ′eutectic reaction. The composition of the solid phase forming from thebulk liquid during solidification varies from the initial bulk liquidcomposition and it continually changes as the temperature decreases.

The breakdown of single crystal solidification is often attributed tothe presence of elevated levels of dense refractory elements thatpartition strongly to either the solid or liquid phase duringsolidification and ultimately result in the formation of freckle chains.Additions of Re and W partition strongly to the dendritic regions duringsolidification, thus depleting the liquid solute of these dense elementsas solidification progresses. This gives rise to large densityimbalances between the bulk liquid and the less dense solute containedwithin the dendritic mushy zone. The compositional differences lead tothe formation of convective instabilities that create solute-rich plumeswhich solidify as channels of equiaxed grains, or freckles. Researchinto solid-liquid elemental partitioning during solidification hasdemonstrated the importance of Ta, W and Re segregation in promoting theformation of these grain defects.

SUMMARY OF THE INVENTION

According to a first aspect of this invention, there is provided acomposition of matter comprising:

-   -   5 wt % to 8 wt % rhenium;    -   4 wt % to 8 wt % tantalum;    -   2 wt % to 5 wt % tungsten;    -   2 wt % to 5.5 wt % molybdenum;    -   2 wt % to 5 wt % chromium;    -   1 wt % to 6 wt % ruthenium;    -   2 wt % to 8 wt % cobalt;    -   5 wt % to 7 wt % aluminium;    -   0 wt % to 2 wt % titanium;    -   0 wt % to 0.5 wt % hafnium;    -   and the balance comprising nickel.

The composition of matter may comprise 5 wt % to 7 wt % rhenium. Thecomposition of matter may comprise greater than 6 wt % rhenium.

The composition of matter may comprise 3.5 wt % to 5 wt % tungsten. Thecomposition of matter preferably comprises less than 4 wt % tungsten.

The composition of matter may comprise 3.5 wt % to 4.5 wt % molybdenum.The composition of matter may comprise greater than 4.5 wt % molybdenum.The composition of matter may comprise less than 2.9 wt % molybdenum.

The composition of matter may comprise 3 wt % to 4 wt % chromium.

The composition of matter may comprise 2 wt % to 6 wt % ruthenium,preferably 3 wt % to 5 wt % ruthenium. The composition of matter maycomprise greater than 4 wt % ruthenium.

The composition of matter may comprise 3 wt % to 8 wt % cobalt.

The composition of matter may comprise 5 wt % to 6.5 wt % aluminium.

The composition of matter may comprise 0.05 wt % to 0.5 wt % hafnium.

The composition of matter may comprise 0.1 wt % to 2 wt % titanium.

In one embodiment the composition of matter may comprise:

-   -   5 wt % to 7 wt % rhenium;    -   4 wt % to 8 wt % tantalum;    -   3.5 wt % to 5 wt % tungsten;    -   3.5 wt % to 4.5 wt % molybdenum;    -   3 wt % to 4 wt % chromium;    -   3 wt % to 8 wt % cobalt;    -   5 wt % to 6.5 wt % aluminium;    -   0 wt % to 0.5 wt % hafnium;    -   0 wt % to 2 wt % titanium;    -   3 wt % to 5 wt % ruthenium;    -   0.1 wt % to 2 wt % titanium;    -   0.05 wt % to 0.5 wt % hafnium;    -   and the balance comprising nickel.

One or more formulations of this embodiment may comprise less than 4 wt% tungsten. One or more formulations of this embodiment may comprisegreater than 4 wt % tungsten.

One or more formulations of this embodiment may comprise greater than 4wt % ruthenium. One or more formulations of this embodiment may compriseless than 4 wt % ruthenium.

The composition of matter is preferably a superalloy, desirably a nickelbased superalloy.

According to a second aspect of this invention, there is provided asingle crystal article formed from a composition of matter as describedabove.

Preferably the article is an aerofoil. The article may be an aerofoilblade, preferably a turbine blade.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of exampleonly with reference to the accompanying drawings, in which:

FIG. 1 are graphs of the effect of Cr and Mo additions on thesolidification paths of W and Re in C17;

FIG. 2 is a photo of an as-cast turbine blade illustrating the fourareas for documentation of solidification related defects arising duringthe casting trials;

FIG. 3 are graphs illustrating the effects of Cr and Mo additions bycomparing the solidification paths of W and Re an UCSX6 and UCSX7;

FIG. 4 shows photos of the macroetched platform of the as-cast turbineblades of (a) UCSX6 (b) UCSX7 and (c) UCSX8 showing a decrease in thenumber of freckle defects from left to right;

FIG. 5 shows optical micrographs of the as-cast microstructures of (a)C17 (b) C17+Mo (c) C17 and (d) C17+Cr+Mo showing decreasing levels ofinterdendritic eutectic as Mo and Cr are added;

FIG. 6 is a graph of quantitative analysis showing a decreasing volumefraction of eutectic with increasing Mo and Cr contents in C17; and

FIG. 7 is a graph of the effect of Ir additions on the partitioningbehaviour of Ta in C17.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

One feature of the preferred embodiment of the present invention is thespecific combination of high Cr and Mo contents complemented by a low(W+Re)/Ta ratio to reduce the susceptibility of the nickel-basedsuperalloy to solidification related defects during single crystalsolidification.

It is known from the prior art that some elements, such as W and Re, aredetrimental to the ease of single crystal solidification. The beneficialeffect of Cr and Mo additions in decreasing the severity of thesolid-liquid partitioning of elements such as W and Re, was noted in theanalysis of the as-cast structures of forty-seven Ni-base superalloycompositions to assess the influence of the constituent elements ontheir solidification characteristics. The compositions in wt % ofselected alloys investigated are listed in Table 1.

A Cameca SX-100 electron microprobe with five wavelength dispersivespectrometers (WDS) was used to quantify the degree to which theconstituent elements of all thirty-nine alloys segregate duringsolidification. A 15 by 15 point grid was used over a 1 mm² area of thepolished sample surface with a 10 second collection time per peak foreach element in addition to a 5 second background measurement eitherside of the peak. This provided representative compositional informationfrom the dendrite cores to the interdendritic regions of thenon-equilibrium solidified alloys. The solid-liquid partitioncoefficients, or k values (where k=X_(S)/X_(L))i for each element werethen quantified using a modified Scheil-analysis. The Scheil equationis:Xs=kX ₀(1−f _(s))^((k−1))Where X_(s) is the mole fraction of solute in the solid, f_(s) is thevolume fraction solid, X₀ is the nominal composition, or in thisinstance the average composition of each element as determined by theelectron microprobe. The degree of segregation is related to themagnitude of the partition coefficient. No segregation occurs when k=1while coefficients greater and less than unity indicate that thecorresponding element is partitioned preferentially to the solid andliquid respectively during solidification. The compositional data forthe elements in each alloy measured at each point in the electron probemicroanalysis was arranged into ascending or descending order dependingon whether the element partitioned preferentially to the dendrite coreor to the interdendritic regions. This was then plotted as a function ofthe volume fraction solid. To determine the solid-liquid partitioncoefficients, k, for each of the constituent elements, the Scheilequation was fitted to the experimental data and then the value of k wasadjusted until the best fit was achieved.

For the majority of the SRR300 series of alloys, levels of Co, A1 and Hfwere held constant while levels of Cr, Mo, W, Re and Ta weresystematically varied to investigate the effects of these additions onthe resulting solidification characteristics. For example, to identifythe effect of increasing Re and W additions, SRR300A was doped with 1.2wt % Re (SRR300B), 1.2 wt % W (SRR300C) and 0.6 wt % Re+0.6 wt % W(SRR300D). The influence of Co on the solid-liquid partitioncoefficients of the other major alloying elements was investigated withalloys SRR300J, SRR300D and SRR300K where Ni was substituted byincreasing Co contents ranging from 2, to 8, to 12 wt % respectively.Four of the alloys, RR3010, SRR300B, SRR300C and SRR300D were doped with1 and 3 wt % (0.6 and 1.9 at %) ruthenium. Prior art has demonstrated Ruto be a potentially beneficial alloying element that is capable ofstabilising the microstructure against the formation of topologicallyclose-packed phases at elevated temperatures. It was thereforeconsidered important to observe whether such additions would bedetrimental to the solidification characteristics of the alloy. Includedwithin these experimental alloys are nine high refractory Ru-containingsingle crystal alloys with the UCSX prefix. UCSX2 includes threevariants of increasing Ru content ranging from 2 to 5 wt % at theexpense of Ni to negate dilution effects. UCSX6, UCSX7 and UCSX8 weredesigned for the purposes of casting trials and are detailed later inthe document. A simplified Ni-base superalloy, C17, with constant levelsof Co, W, Re, and Ta was used to isolate the effects of A1, Cr, Mo andIr additions on the segregation behaviour of the constituent elements.While prior art has also demonstrated that Ir is a potentmicrostructural stabilising element, in this research Ir was added toinvestigate how the presence of another element, which also partitionspreferentially to the growing solid during solidification, influencesthe relative severity of segregation of W and Re. Finally eight alloys,with the LDSX prefix, were investigated to explore the benefits of lowW, with a combination of high Cr, Mo and Ru on castability.

The results from the electron probe microanalysis are summarised inTable 2. Statistical fluctuations associated with the modified-Scheilanalysis are all within an average deviation of 0.05 pf the partitioncoefficients reported. Generally, the more strongly segregating theelement, the greater the average deviation. Consistent with otherinvestigations, the high-density refractory elements, Ta, W and Re werefound to segregate most severely during solidification i.e. their kvalues were furthest from unity. The degree to which each of theseelements partitioned however, varied significantly with composition overthe range of experimental alloys analysed. Relative changes in both theMo and Cr alloying levels were found to have the most significanteffect. The presence of Mo and Cr in the SRR300 series of alloys wasfound to decrease the extent of segregation for the dense refractoryelements know to promote freckle defects. For example, alloy SRR3001,which has the highest overall content of Cr+Mo, exhibits a substantiallylower degree of segregation when compared to alloys SRR300L and SRR300H,which have similar levels of refractory alloying additions by lowerCr+Mo levels. With an intermediate level of Cr+Mo, the measuredsegregation of Re, W and Ta in SRR300B is moderate when compared toSRR300I, SRR300L and SRR300H.

It is difficult to isolate the effects of Cr and Mo additions within theSRR300 series of alloys, since changes in their concentrationsthroughout the alloy compositions investigated are companied bysignificant changes in other important alloying elements. Hence, in anattempt to isolate their effects, additions of 4.5 wt % Cr and 2.2 wt %Mo were systematically made to the experimental Ni-based single crystalalloy containing no Cr and Mo additions, TABLE 1 Compositions (in wt. %)of selected alloys analysed. Alloy Ni Al Cr Co Mo Ti Nb Ta W Re Ru Ir HfRR3010 Bal. 5.9 1.7 3.1 0.5 0.1 0.1 8.5 5.5 6.8 — — — RR3010 + 1 Ru 5.81.7 3.1 0.5 0.1 0.1 8.4 5.4 6.7 1.0 — — RR3010 + 3 Ru Bal. 5.7 1.7 3.00.5 0.1 0.1 8.3 5.3 6.6 3.0 — — SRR300A Bal. 5.8 4.0 8.0 2.2 — — 7.5 4.64.1 — — 0.1 SRR300B Bal. 5.8 4.0 8.0 2.2 — — 7.5 4.6 5.2 — — 0.1SRR300B + 1 Ru Bal. 5.7 4.0 7.9 2.2 — — 7.4 4.6 5.2 1.0 — 0.1 SRR300B +3 Ru Bal. 5.6 3.9 7.8 2.1 — — 7.3 4.5 5.1 3.0 — 0.1 SRR300C Bal. 5.8 4.08.0 2.2 — — 7.5 5.8 4.1 — — 0.1 SRR300C + 1 Ru Bal. 5.7 4.0 7.9 2.2 — —7.4 5.7 4.1 1.0 — 0.1 SRR300C + 3 Ru Bal. 5.6 3.9 7.8 2.1 — — 7.3 5.64.0 3.0 — 0.1 SRR300D Bal. 5.8 4.0 8.0 2.2 — — 7.5 5.2 4.7 — — 0.1SRR300D + 1 Ru Bal. 5.7 4.0 7.9 2.2 — — 7.4 5.1 4.7 1.0 — 0.1 SRR300D +3 Ru Bal. 5.6 3.9 7.8 2.1 — — 7.3 5.0 4.6 3.0 — 0.1 SRR300E Bal. 5.8 4.08.0 2.2 — 0.6 6.5 5.2 4.7 — — 0.1 SRR300G Bal. 5.8 4.0 8.0 3.5 — 0.6 6.54.0 4.0 — — 0.1 SRR300H Bal. 5.8 4.0 8.0 — — — 7.5 7.0 5.3 — — 0.1SRR300I Bal. 5.8 5.5 8.0 2.0 — — 7.5 4.0 5.3 — — 0.1 SRR300J Bal. 5.84.0 2.0 2.2 — — 7.5 5.2 4.7 — — 0.1 SRR300K Bal. 5.8 4.0 12.0 2.2 — —7.5 5.2 4.7 — — 0.1 SRR300L Bal. 6.0 2.5 8.0 2.2 — — 7.5 4.6 5.3 — — 0.1C17 Bal. 6.0 — 12.0 — — — 6.0 9.3 6.0 — — — C17 + A1 Bal. 6.5 — 11.9 — —— 6.0 9.3 6.0 — — — C17 + Cr Bal 5.7 4.5 11.5 — — — 5.7 8.9 5.7 — — —C17 + Mo Bal. 5.9 — 11.7 2.2 — — 5.9 9.1 5.9 — — — C17 + Cr + Mo Bal.5.6 4.5 11.7 2.2 — — 5.6 8.7 5.6 — — — C17 +1 at. % Ir Bal. 5.8 — 11.7 —— — 5.8 9.0 5.8 — 3.0 — C17 +3 at. % Ir Bal. 5.5 — 11.0 — — — 5.5 8.65.5 — 8.6 — UCSX2 + 2 Ru Bal. 5.4 3.0 8.0 1.0 — — 8.0 8.0 6.5 2.0 — 0.1UCSX2 + 3 Ru Bal. 5.4 3.0 8.0 1.0 — — 8.0 8.0 6.5 3.0 — 0.1 UCSX2 + 5 RuBal. 5.4 3.0 8.0 1.0 — — 8.0 8.0 6.5 5.0 — 0.1 UCSX6 Bal 6.3 — 4.0 — — —6.0 8.0 6.8 3.0 — — UCSX7 Bal 6.0 1.5 4.0 3.0 — — 6.0 8.0 6.8 3.0 — —UCSX8 Bal 5.7 1.5 6.0 3.0 — — 8.0 6.0 6.0 3.0 — — LDSX1 Bal. 6.0 3.0 3.02.5 0.25 — 6.5 2.9 6.2 3.5 — 0.1 LDSX2 Bal. 6.0 3.0 8.0 5.0 0.25 — 6.52.9 6.2 3.5 — 0.1 LDSX3 Bal. 6.0 3.0 3.0 5.0 0.25 — 6.5 4.8 6.2 3.5 —0.1 LDSX4 Bal. 6.0 3.0 8.0 2.5 0.25 — 6.5 4.8 6.2 3.5 — 0.1 LDSX5 Bal.6.0 3.0 8.0 2.5 0.25 — 6.5 2.9 6.2 5.0 — 0.1 LDSX6 Bal. 6.0 3.0 3.0 2.50.25 — 6.5 4.8 6.2 5.0 — 0.1 LDSX7 Bal. 6.0 3.0 3.0 5.0 0.25 — 6.5 2.96.2 5.0 — 0.1 LDSX8 Bal. 6.0 3.0 8.0 5.0 0.25 — 6.5 4.8 6.2 5.0 — 0.1C17. The partition coefficients of the dense refractory elements in thebase C17 alloy are far from unity and are comparable to RR3010 (Table2), which is also a low Cr, low Mo content alloy. While the improvementsin the solid-liquid partition coefficients of W and Re upon addition ofCr and Mo to C17 are not as dramatic as observed in the SRR300 alloyseries, the trends are nonetheless consistent with the prior findings,emphasising the decrease in segregation associated with the overall Crand Mo content present within a given alloy. Thee effects areillustrated more clearly in FIG. 1, where the segregationcharacteristics are clearly being influenced by the presence of Cr andMo. In this particular set of alloys, Mo additions appear more potentthan Cr in suppressing the segregation behaviour of W and Re. Anaddition of 2.2 wt % (1.4 at %) Mo decreases the microsegregation of Wand Re to a greater extend than an addition of 4.5 wt % (5.2 at %) Cr.The largest decrease however, was achieved when both Cr and Mo wereadded to C17. No significant improvements to the partitioning of Ta werenoted upon addition of Cr and Mo.

Ruthenium was found to be largely neutral in its influence on thesolidification characteristics of the alloys. Slightly lower levels ofCr, Mo and Re segregation were measured in alloys containing 1 wt % Ru,however little to no additional improvement accompanied an increase inRu content to 3 wt %. This is consistent with the results for theRu-variants of UCSX2, where any changes in the partition coefficients ofthe constituent elements associated with increasing Ru contents areinsignificant. Ru itself partitions only slightly to the dendrite core,having a k value close to unity.

No significant alteration to the solidification paths of the constituentelements was noted with large changes in Co contents from 2, to 8, to 12wt % in alloys SRR300J, SRR300D and SRR300K respectively. The same wastrue for an increase of 0.5 wt % A1 to the base C17 alloy. Excluding Crand Mo additions, no other elemental additions were revealed tosignificantly influence the partitioning of W and Re. However,increasing the overall concentrations of W and Re leads to more severepartitioning of the elements to the initial fraction solid (compareSRR300A to SRR300B and SRR300C in Table 2). Comparison of thesolid-liquid partition coefficients of W and Re in SRR300E to those inSRR300G in Table 2 shows the benefit of combining a high Moconcentration with lowered W and Re concentrations. The LDSX series ofalloys support this finding. The severity of partitioning of theconstituent elements is lessened through the combination of high Cr andMo contents with a low W content despite maintaining the high Re and Rucontents necessary for enhanced creep properties and microstructuralstability at elevated temperatures.

Having determined the partition coefficients for each composition,multiple linear regression analysis was performed on the experimentaldata in Table 2 to obtain formulae for the prediction of thesolid-liquid partition coefficients of the major constituent elements.The magnitude of the coefficient associated with each of the elements inthe linear regression analysis provides an indication of the relativeinfluences of the other elements on the partitioning of the element inquestion. The regression equations corresponding to the elements know tobe most important in the promotion of grain defects, namely W and Re,show the potential benefits of Cr and Mo additions in decreasing theintrinsic susceptibility of an alloy to single crystal breakdown duringsolidification (note that any coefficients of order 10⁻⁴ and less havebeen omitted):Kw=0.281+0.0988[wt % A1]−0.00316[wt % Cr]+0.0101[wt % Co]−0.0063[wt %Mo]+0.0289[wt % Ta]−0.00325[wt % W]+0.0258[wt % Re]+0.00418[wt % Ru]kR_(e)=1.37+0.0205[wt % A1]−0.0168[wt % Cr]−0.00586[wt % Co]−0.0416[wt %Mo]−0.0035[wt % Ta]+0.0055[wt % W]+0.0192[wt % Re]−0.00461[wt % Ru]

TABLE 2 Measured and calculated solid-liquid partition coefficients ofselected alloys. Alloy Ni Al Cr Co Mo Ta W Re Ru Ir RR3010 0.97 0.881.15 1.08 — 0.77 1.26 1.57 — — RR3010 + 1 Ru 0.98 0.87 1.10 1.08 — 0.761.26 1.53 1.04 — RR3010 +3 Ru 0.98 0.87 1.10 1.08 — 0.76 1.27 1.54 1.04— SRR300A 0.98 0.94 1.04 1.04 1.06 0.80 1.20 1.36 — — SRR300B 0.97 0.921.11 1.06 1.09 0.78 1.21 1.39 — — SRR300B + 1 Ru 0.98 0.91 1.06 1.051.08 0.76 1.22 1.35 1.04 − SRR300B + 3 Ru 0.98 0.91 1.05 1.05 1.07 0.761.21 1.36 1.04 — SRR300C 0.97 0.92 1.11 1.06 1.10 0.78 1.23 1.36 — —SRR300C + 1 Ru 0.98 0.91 1.05 1.06 1.08 0.76 1.23 1.32 1.03 — SRR300C +3 Ru 0.98 0.91 1.04 1.05 1.07 0.77 1.23 1.32 1.03 — SRR300D 0.97 0.911.11 1.07 1.09 0.76 1.24 1.43 — — SRR300D + 1 Ru 0.98 0.91 1.06 1.051.07 0.76 1.24 1.40 1.04 — SRR300D + 3 Ru 0.98 0.91 1.05 1.05 1.07 0.771.23 1.39 1.04 — SRR300E 0.98 0.90 1.08 1.06 1.09 0.77 1.23 1.42 — —SRR300G 0.98 0.95 1.06 1.04 1.08 0.83 1.19 1.27 — — SRR300H 0.97 0.891.06 1.07 — 0.75 1.27 1.47 — — SRR300I 0.98 0.95 1.08 1.04 1.08 0.871.12 1.23 — — SRR300J 0.98 0.91 1.07 1.07 1.08 0.77 1.25 1.43 — —SRR300K 0.97 0.91 1.06 1.06 1.07 0.76 1.24 1.43 — — SRR300L 0.97 0.901.07 1.06 1.08 0.74 1.27 1.48 — — C17 0.95 0.85 — 1.05 — 0.65 1.30 1.55— — C17 + Al 0.95 0.85 — 1.06 — 0.66 1.31 1.55 — — C17 + Cr 0.96 0.851.03 1.05 — 0.64 1.28 1.53 — — C17 + Mo 0.96 0.87 — 1.05 1.10 0.67 1.251.44 — — C17 + Cr + Mo 0.97 0.87 1.03 1.05 1.10 0.67 1.22 1.40 — — C17 +1 at. % Ir 0.95 0.85 — 1.05 — 0.68 1.29 1.55 — 1.12 C17 + 3 at. % Ir0.95 0.85 — 1.04 — 0.76 1.28 1.53 — 1.13 UCSX2 + 2 Ru 0.96 0.86 1.071.06 1.08 0.72 1.26 1.48 1.05 — UCSX2 + 3 Ru 0.96 0.86 1.07 1.06 1.080.72 1.26 1.47 1.05 — UCSX2 + 5 Ru 0.96 0.86 1.07 1.06 1.07 0.72 1.251.47 1.06 — UCSX6 0.95 0.84 — 1.08 — 0.72 1.31 1.63 1.06 — UCSX7 0.960.86 1.09 1.08 1.09 0.73 1.25 1.47 1.06 — UCSX8 0.97 0.88 1.07 1.07 1.060.74 1.23 1.42 1.05 — LDSX1 0.98 0.91 1.09 1.06 1.09 0.81 1.23 1.42 1.05— LDSX2 0.98 0.93 1.06 1.05 1.06 0.80 1.27 1.28 1.07 — LDSX3 0.98 0.911.09 1.07 1.06 0.79 1.21 1.32 1.06 — LDSX4 0.97 0.90 1.07 1.05 1.09 0.761.28 1.40 1.06 — LDSX5 0.98 0.91 1.06 1.05 1.08 0.79 1.29 1.38 1.06 —LDSX6 0.97 0.89 1.09 1.07 1.08 0.78 1.23 1.42 1.05 — LDSX7 0.98 0.921.08 1.06 1.06 0.82 1.22 1.31 1.06 — LDSX8 0.97 0.91 1.06 1.05 1.05 0.771.27 1.29 1.07 —

The coefficients for Cr and Mo in the determination of k_(w) and k_(R)_(e) exert the largest influence in the minimisation of both towardsunity. The coefficient for Mo is greater than that of Cr in bothinstances demonstrating the greater potency of Mo additions inminimising the severity of W and Re segregation.

Casting trials were performed on three alloy compositions UCSX6, UCSX7and UCSX8 (Table 1) specially devised to validate the importance of Crand Mo additions and a low (W+Re)/Ta ratio in minimising the formationof solidification related defects.

The elemental concentrations in each alloy are typical of advancedsingle crystal superalloy compositions and were intentionally designedto investigate whether such alloys could be made more amenable to singlecrystal solidification whilst maintaining the high refractory contentsnecessary to enhance creep resistance. UCSX6, a Cr- and Mo-free alloywith an undesirable (W+Re)/Ta ratio was designed to the most prone tosolidification defects while UCSX8, with reduced amounts of Re and W,which partition preferentially to the growing solid duringsolidification, and complementing this with increased Ta contents, whichfurther decreases the density inversion by partitioning preferentiallyto the bulk liquid, was designated to be the least prone. UCSX7 wasdesigned to experimentally verify the potential benefits of Cr and Moadditions on the severity of the solid-liquid partitioning of Re and W.The sole difference between UCSX6 and UCSX7 is the addition of 1.5 wt %Cr and 3.0 wt % Mo and the amount of Al was adjusted to ensure the sameγ′ volume fraction as predicted by the JMatPro software for UCSX6.

A production scale Bridgman furnace was used to simultaneously solidifyfive solid turbine blades in a ceramic cluster mould at constantprocessing conditions using a withdrawal rate of 230 mm per hour. Theas-cast crystals were subsequently macroetched to reveal the presence,location and number of macroscopic grain defects, such as freckle chainsand misoriented grains, on the surface of the castings. To document thelocation of any defects, the blade was separated into four areas: thetip, blade, platform and root (FIG. 2). For every blade of eachcomposition the number of defects in each area were counted and averagedover the five blades for subsequent comparison.

The results were listed in Table 2 and Table 3 experimentally verifyboth the beneficial effect of Cr and Mo additions and the importance ofmaintaining a low (W+Re)/Ta ratio. TABLE 3 Number, location and totalnumber of solidification related defects in each alloy tested in thecasting trails. Location UCSX6 UCSX7 UCSX8 Tip 7.5 ± 1.0 0 0 Blade 1.5 ±0.5 0 0 Platform 16.3 ± 1.5 10.8 ± 1.7 3.0 ± 1.4 Root 18.8 ± 1.3  9.3 ±1.9 0.8 ± 0.5 Total No. of Defects 44.0 ± 1.4 20.0 ± 1.2 3.8 ± 1.3

The Cr- and Mo-free UCSX6 alloy with the most severe solid-liquidpartitioning of the constituent elements was found to be the most proneto freckle formation. The total number of freckle defects in UCSX6 wasmore than halved in UCSX7 solely due to the addition of 1.5 wt. % (1.9at. %) Cr and 3.0 wt. % (2.0 at. %) Mo. The change in the solidificationpaths of W and Re as a result of these additions is illustrated in FIG.3. Manipulation of the (W+Re)/Ta ratio in UCSX8 resulted in furtherreductions in the solid-liquid partitioning coefficients andconsequently substantially fewer defects.

In all three alloys, freckle defects were concentrated in the platformand root of the casting while only in UCSX6, designed to be the mostsusceptible of the three alloys to single crystal breakdown, were anyfreckle defects observed in the tip and blade. The macroetched platformsof the as-cast blades of UCSX6, UCSX7 and UCSX8 in FIG. 4 show adecrease in the number of freckle defects from UCSX6 to UCSX8.

Freckle formation occurs when the driving force for fluid flow, asdescribed by the destabilising buoyancy forces corresponding to thesolute-induced density inversion term Δρ/ρo exceeds the surroundingfrictional forces. Hence, by reducing the amount by which W and Re aredepleted from the interdentritic solute during solidification, Cr and Moadditions decrease the potential for density inversion and, in so doing,lessen the susceptibility of the alloy to the formation of localisedconvective instabilities.

To explain the effect of Cr and Mo on the solid-liquid partitioning ofRe and W the principal factors which control the solidificationcharacteristics of an alloy need to be considered, namely the phasespresent in the as-cast microstructure, the freezing range and theoverall thermodynamics of the system. During directional solidificationunder steady state conditions, the mushy zone is comprised of singlephase y dendrites and liquid solute. Since the alloying additions inNi-base superalloys tend to partition preferentially into either the γor γ′ phrases, limited solubility of γ′ forming elements exists withinthe single phase dendrites during solidification. Hence, elements suchas Ta, A1 and Hf become enriched into the liquid solute. The otheralloying additions, Re, W, Cr, Co, Mo and Ru, are soluble in the γ phaseand tend to partition preferentially to different degrees into the γdendrites during solidification. A reduction in the degree ofmicrosegregation could occur if the respective alloying additionsshifted the overall composition of the alloy closer to that of the γ/γ′eutectic. An initial composition further from the eutectic compositionwould enable segregation over a greater freezing range prior toattainment of the eutectic composition, at which point the remainingliquid would solidify as eutectic and no further solid-liquidpartitioning could take place. However, both Cr and Mo additions wereshown to decrease the degree of segregation (FIG. 3) and the volumefraction of eutectic in the as-cast condition (FIG. 5 and FIG. 6). FIG.5 reveals the dendritic as-cast structures for the C17 alloy series.Pools of γ/γ′ eutectic dispersed in the interdendritic regions areclearly distinguishable within the dendritic structure. Qualitativeexamination of the as-cast microstructures of each alloy set indicatedthat the γ/γ′ eutectic content decreased with increasing Cr and Mocontents (FIG. 5(a)-(d)). In the base alloy a continuous distribution ofeutectic around the dendrites is evident whereas the eutectic poolsbecome more isolated and dispersed as the overall content of alloyingadditions increases. This trend was confirmed quantitatively (FIG. 6),where the volume fraction eutectic decreased at a rate comparable to theoverall amount of alloying addition. For example, an addition of 4.5 wt% Cr decreased the eutectic content to a greater extend than an additionof 2.2 wt % Mo, while the least amount of eutectic was present in thealloy containing 6.7 wt % (4.5 wt % Cr+2.2 wt % Mo) of alloyingadditions. The decrease in eutectic volume fraction in the C17 alloyswas not unexpected since doping of the alloys with Cr and Mo effectivelydiluted the system, thus drawing the composition of the alloy furtherfrom that of the eutectic. In addition, Cr and Mo are primarily γ ratherthan γ′ formers so it would be unlikely that they would promote eutecticformation.

The effect of Cr and Mo on the temperature range over which segregationcould occur was also investigated by measuring the solidus and liquidustemperatures of the C17 base alloy and the Cr- and Mo-containingcounterparts using Differential Scanning Calorimetry (DSC). Since themagnitude of the freezing range (T_(S)-T_(L)) governs the extent of themushy zone during directional solidification, minor changes could alsoinfluence the segregation characteristics of the alloy. Alloys thatsolidify over a relatively small freezing range may exhibit minimallevels of W and Re segregation since the thermal fields are likely tohave a larger influence than the solute fields during solidification.The results however, show that the freezing range is narrowest for theundoped C17 base alloy (Table 4). Increases of ˜7° C. were observed with2.2 wt % Mo additions while Cr additions to C17 increased the freezingrange by ˜20° C. The alloy containing both elemental additions exhibitedthe largest freezing range. Coupled with the microstructuralobservations regarding the volume fraction of eutectic, results fromthis study indicate that the solidification characteristics are stronglydependent upon alloy composition. TABLE 4 DSC results showing the effectof Cr and Mo additions on the freezing range of C17. Solidus LiquidusFreezing Range Alloy (° C.) (° C.) (° C.) C17 1399 1420 21 C17 + Mo 13921420 28 C17 + Cr 1381 1422 41 C17 + Cr + Mo 1374 1420 46

As Ni-base superalloys become more heavily alloyed, supersaturation ofthe γ phase with Re, W, Co, Cr, Mo and Ru during solidification couldoccur as the solubility limits are exceeded. Elements that tend toincrease the liquidus temperature of Ni (Re and W), also tend tosegregate most strongly to the single phase γ dendrites duringsolidification. Ru is an unusual alloying addition as it slightlyincreases the liquidus temperature, but segregates only mildly to the γphase. Other γ forming elements, Co, Cr and Mo, tend to slightly lowerthe solidus temperature and segregate only moderately to the γ phaseduring solidification. Based on the observed changes in microstructureand associated changes in freezing range, compositional changesassociated with the Cr and Mo additions appeared to be altering thesolid-solution solubility limits that govern segregation duringsolidification. In general, higher levels of refractory elementsegregation were measured in alloys containing low overall levels of thepotent γ forming elements. Based on atomic percentages, the lowestcombined levels of Re, W, Cr and Mo from the initial study were found inRR3010 and C17, 6.6% and 5.25% respectively. These alloys also exhibitedthe largest degree of segregation, with K_(R) _(e) of 1.57 and 1.55 andk_(w) of 1.26 and 1.30 for RR3010 and C17 respectively. Alloys, such asSRR300G and SRR3001, which contain significantly higher levels (9.6 and10.9 at % respectively) of these potent γ forming elements tend toresult in less segregation during solidification (K_(R) _(e) is 1.28 and1.23 and k_(w) is 1.19 and 1.12 for SRR300G and SRR300I respectively).This was investigated further using 1 and 3 at % (3 and 9.6 wt %)iridium additions to C17. Despite Ir partitioning to the solid morestrongly than either Cr or Mo in the same alloy system (Table 2) and thedoping concentrations being greater than the amount to which Mo wasadded to C17, no significant changes in the solidification paths ofeither W or Re was noted. Interestingly however, Ir greatly reduced thesolid-liquid partitioning of Ta (FIG. 7). The Ir—Ta binary phase diagramindicates extensive interactions between the two elements, including theformation of a phase over a wide composition range, the sameintermetallic phase observed in the binary phase diagrams of Cr—Re,Mo—Re and W—Re. No such interactions are observed for the elements (Co,A1 and Ru) which exhibited a negligible influence on the solidificationpaths of W and Re. While it is an over simplification to compare binarywith multi-component systems, the fact that Cr, Mo, W and Re all combineto form thermodynamically stable intermetallic topologicallyclose-packed (TCP) phases in multi-component alloys at elevatedtemperatures suggest that these strong interactions extend tomulti-component systems.

The electron configurations of Cr, Mo, W and Re are listed in Table 5.Atomic Element Number Electron Configuration Cr 24 1s² 2s² 2p⁶ 3s² 3p⁶3d⁵ 4s¹ Mo 42 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s¹ W 74 1s² 2s² 2p⁶3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d⁴ 6s² Re 75 1s² 2s² 2p⁶ 3s² 3p⁶3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d⁵ 6s²

While not wishing to be limited to a particular theory, it is thought tobe the electron vacancies in the d-shell orbitals (highlighted in bold)which provide the high potential for the formation of strong TCP phasebonds between these atoms. The d electrons are loosely bound and becomedelocalised together with s electrons resulting in stronger attractionsdue to the involvement of more electrons. The inherent stability of TCPsindicates there may be extensive covalent bonding interactions via the delectrons and orbitals of the atoms of the TCP forming elementssupplementing their metallic bonding. While no long range orderingexists in the liquid state, a limited degree of short range ordering mayexist between these elements. Hence, it is thought that the affinitythese elements have for one another in the solid state persists in themelt and consequently the solidification paths of W and Re are alteredthrough their interactions with Cr and Mo atoms. The greater potency ofMo as compared to Cr additions in reducing the partitioning of W and Recan also be explained through these d shell interactions. The 4dorbitals of Mo atoms are much more extended than the 3d orbitals of Crrelative to the filled s and p orbitals of the same shell. This isbecause the nuclear charge is increased from Cr to Mo, meaning the s andp filled subshells, which feel the increased nuclear charge morestrongly due to their orbits running much closer to the nucleus, cannotexpand as much as the 4d orbital thus allowing for greater overlap ofthe 4d orbitals with the d orbitals of neighbouring atoms resulting ingreater interaction. This is exemplified by the wider composition rangeover which σ phase is stable in the Mo—Re binary phase diagrams comparedto that of Cr—Re. The exact mechanisms by which these interactionsdecrease the extent of microsegregation of W and Re duringsolidification is unclear. The effect may be both thermodynamic andkinetic in nature. It is possible that these interactions cause Cr andMo atoms to form metastable clusters with W and Re within the melt whichincrease the thermodynamic stability of the liquid with respect to thesolid, particularly in the vicinity of the crystallisation temperaturewhere volume differences between the solid and liquid are small and theatomic arrangements in the liquid are consequently more or less similarto their arrangements in the corresponding solid bodies. Kinetically,these same clusters could effectively lower the liquid diffusivities ofW and Re, thus reducing the rate at which W and Re atoms can diffusetowards the solid-liquid interact. The result is a compromise of thesolid-liquid partition coefficients of each participant element in thecluster; that is the coefficients of W and Re are reduced and those ofCr and Mo are increased. This is best illustrated by comparison of thesolidification characteristics of UCSX6 to UCSX7 and SRR300A to SRR300Bin Table 2. The addition of Cr and Mo to UCSX6 results in the reductionof the k values of W and RE closer to those of Cr and Mo whereas theincrease in partitioning of Re in SRR300B, associated with a higher Recontent, results in a corresponding increase in the partitioning of Crand Mo to the growing solid K_(C) _(r) is 1.04 and 1.11 and K_(M) _(o)is 1.06 and 1.09 in SRR300A and SRR300B respectively). This compromiseis beneficial to the overall castability of the alloy due to the higherdensity of W and Re compared to Cr and Mo.

Despite quantification of the mechanisms by which Cr and Mo influencethe solid-liquid partitioning of W and Re, the observed effects of Moand Cr are interesting particularly considering the recent trendstowards developing low chromium, low molybdenum content superalloys,where for example in RR3010 the Mo and Cr contents are now at 0.5 and1.7 wt % respectively. This trend is primarily associated with thedestabilising effect that Cr and Mo have on the γ-γ′ microstructureafter prolonged exposures at elevated temperatures. Minimisation ofthese elements have enabled advanced Ni-based single crystals to bealloyed with increasing levels of potent solid solution strengtheners,such as W and Re, to enhance the high temperature creep properties. Cris typically maintained at sufficient concentrations to provide acertain level of hot corrosion and oxidation resistance. These changeshowever, appear to be detrimental to the manufacture and production ofadvanced single crystal components because of the increased likelihoodfor grain defect formation during directional solidification. As theoverall content of Re has increased the price of bar stock, improvementsin yield are of greater importance. The results from this investigationdemonstrate that the intrinsic tendency of high refractory Ni-basedsingle crystal alloys to form grain defects during solidification isdecreased by increasing the overall level of Cr and Mo in the alloy.Both of these elements reduce the extent of microsegregation of thedense refractory elements W and Re during solidification known to causesingle crystal breakdown. In fact, this beneficial effect appears asthough it is due to the very fact that both Cr and Mo are TCP formingelements. As a result, increasing the levels of both Cr and Mo toimprove the solidification characteristics of single crystal Ni-basesuperalloys will further destabilise the microstructure. However, theresulting microstructural instabilities could potentially be controlledthrough the addition of ruthenium. Preliminary studies suggest that Rucan be added in the concentration necessary to improve microstructuralstability without significant detriment to the solidificationcharacteristics of the alloy. Moreover, the decreased level ofmicrosegregation accompanying increases in Mo and Cr contents wouldreduce the extent of local Re supersaturation in the as-cast crystalsand enable homogenisation to occur more rapidly during solution-heattreatment.

Any practical single crystal alloy has to have a combination of usefulproperties. These properties include alloy density, creep-ruptureproperties, high temperature strength and fatigue resistance,microstructural stability and oxidation and hot corrosion resistance,together with acceptable raw material and processing costs. Increasingthe mechanical strength nearly always involves additions which are bothcostly and dense, so in order to maintain an acceptable component cost,the processing costs must be kept as low as possible.

This invention is linked to one important aspect of processing cost, theyield of acceptable castings, commonly called castibility. The novelaspect of this invention is the identification of relatively highchromium and molybdenum contents as beneficial, in combination with thewell-known damaging effects of rhenium and tungsten, and beneficialeffect of tantalum. These considerations lead to the claimed compositionranges in the following way.

The chromium content, preferably 3 to 4 weight percent, should not beless than about 2 weight percent nor more than about 5 weight percent(all compositional percentages herein are by weight). The chromiumcontent is desirably high due to its benefit on both hot corrosion andoxidation resistance and castability by minimising the formation ofsolidification related grain defects. However it desirably does notexceed 5% because chromium contributes to microstructural instabilitywith respect to the formation of deleterious topologically close-packed(TCP) phases following prolonged exposure at elevated temperatures.Below 2% chromium the hot corrosion and oxidation resistance becomeunacceptable in the preferred embodiment.

The molybdenum content ranges from 2 to 5.5%, and preferably 3.5 to4.5%. Molybdenum is preferably present in concentrations greater than 2%because it improves the castability of the alloy. It is also aneffective strengthening element in the gamma phase and has a lowerdensity than the alternative strengtheners tungsten and rhenium. Anupper limit of 5.5% is desirable in the preferred embodiment becausemolybdenum destabilises the microstructure leading to precipitation ofdamaging TCP precipitates.

Ruthenium is present in the preferred embodiment in an amount of atleast 1%, conveniently, from about 1 to about 6%, desirably 2 to about6%, and preferably from about 3 to about 5%. Ruthenium provides strengthand stabilises the microstructure with respect to the formation of TCPphases, so counteracting the effect of the necessary elevated chromiumand molybdenum contents for improved castability. In this work rutheniumwas found to be largely neutral in its effect on castability; highlevels of ruthenium can therefore be added for strength and stabilitywithout compromising casting yield. A preferred upper limit is about 6%,due to the expense of ruthenium additions.

Tungsten contents range in the preferred embodiment, from about 2 toabout 5%, preferably 3.5 to 5%. Tungsten partitions to both the gammaand gamma prime phases and is also an effective strengthener.Concentrations greater than 2% are desirable to provide sufficientstrength to the superalloy but its density undesirably increases thedensity of the alloy and greatly hinders the ease of single crystalsolidification. Its content must therefore by maintained below about 5%to ensure a good casting yield by minimising the (W+Re)/Ta ratio. Theselower levels of tungsten will also improve the microstructural stabilityand oxidation and hot corrosion resistance of the alloy.

Rhenium is present in an amount of from about 5 to about 8%, preferablyfrom about 5 to about 7%. Concentrations of rhenium above 5% aredesirable to achieve the high temperature strength, particularly whencoupled with a relatively low tungsten content, since it is a potentsolid-solution strengthening element of the gamma phase. Rhenium shouldnot be added in amounts greater than about 8% to the preferredembodiments because it is a dense and expensive element, is detrimentalto castability and promotes the formation of TCP phases.

Tantalum is present in an amount of from about 4 to 8%, preferably 5.5to 7%. Tantalum is desirable in concentrations greater than 4% becauseit strengthens the gamma prime phase, provides resistance to hotcorrosion but, most notably in this invention, reduces the formation ofsolidification related grain defects by minimising the (W+Re)/Ta ratio.If the tantalum content is above 8% however, then density of the alloyis undesirably increased. The novel benefits of chromium and molybdenum,coupled with low tungsten mean that tantalum can be reduced to achieve areduced alloy density.

The preferred embodiments of the present invention provide anickel-based single crystal superalloy which have the advantage ofexhibiting improved castability, i.e. less susceptibility to theformation of solidification related grain defects during single crystalsolidification, by increasing the Cr and Mo contents and minimising the(W+Re)/Ta ratio by decreasing the W content relative to typicalnickel-based single crystal superalloy compositions.

The preferred embodiment comprises a nickel-based single crystalsuperalloy where the compositions consists of 2-8 of Co, 2-5 of Cr,2-5.5 of Mo, 2-5 of W, 5-7 of A1, 4-8 of Ta, 5-8 of Re, 2-6 of Ru, 0-2of Ti and 0-0.5 of Hf in terms of % by weight and residual partsubstantially consists of Ni wherein said alloy may contain unavoidableimpurities.

1. A composition of matter comprising: 5 wt % to 8 wt % rhenium; 4 wt %to 8 wt % tantalum; 2 wt % to 5 wt % tungsten; 2 wt % to 5.5 wt %molybdenum; 2 wt % to 5 wt % chromium; 2 wt % to 6 wt % ruthenium; 2 wt% to 8 wt % cobalt; 5 wt % to 7 wt % aluminium; 0 wt % to 2 wt %titanium; 0 wt % to 0.5 wt % hafnium; and the balance nickel andincidental impurities.
 2. A composition of matter according to claim 1comprising 5 wt % to 7 wt % rhenium.
 3. A composition of matteraccording to claim 1 comprising greater than 6 wt % rhenium.
 4. Acomposition of matter according to claim 1 comprising 3.5 wt % to 5 wt %tungsten.
 5. A composition of matter according to claim 1 comprisingless than 4 wt % tungsten.
 6. A composition of matter according to claim1 comprising 3.5 wt % to 4.5 wt % molybdenum.
 7. A composition of matteraccording to claim 1 comprising less than 2.9 wt % molybdenum.
 8. Acomposition of matter according to claim 1 comprising greater than 4.5wt % molybdenum.
 9. A composition of matter according to claim 1comprising 3 wt % to 4 wt % chromium.
 10. A composition of matteraccording to claim 1 comprising 3 wt % to 5 wt % ruthenium.
 11. Acomposition of matter according to claim 1 comprising greater than 4 wt% ruthenium.
 12. A composition of matter according to claim 1 comprising3 wt % to 8 wt % cobalt.
 13. A composition of matter according to claim1 comprising 5 wt % to 6.5 wt % aluminium.
 14. A composition of matteraccording to claim 1 comprising 0.05 wt % to 0.5 wt % hafnium.
 15. Acomposition of matter according to claim 1 comprising 0.l wt % to 2 wt %titanium.
 16. A single crystal article formed from a composition ofmatter according to claim
 1. 17. A single crystal according to claim 16in the form of an aerofoil.
 18. A single crystal article according toclaim 16 in the form of aerofoil blade.
 19. A single crystal articleaccording to claim 17 in the form of a turbine blade.